Nowadays, laminated films having an ethylene-vinyl alcohol copolymer layer have been used in applications such as packaging materials for food, medical applications and the like, taking advantages of their superior gas barrier properties, stretchability, thermoformability and the like. Recently, for the purpose of improving various types of performances such as gas barrier properties, various multilayered structures have been proposed in which a plurality of resin-layers are laminated, each layer having a thickness of micron or submicron order.
Conventional multilayered structures developed so far in which a plurality of resin-layers of an ethylene-vinyl alcohol copolymer are laminated include, for example: (1) an elastomeric barrier film in which at least ten layers of a microlayer polymer composite formed from a fluid barrier material such as an ethylene-vinyl alcohol copolymer, and an elastomer material such as thermoplastic polyurethane are alternately laminated (see Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2002-524317); (2) multilayer film alternately including layers of a hard polymer material such as an ethylene-vinyl alcohol copolymer and flexible polymer material (see Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2003-512201), and the like.
However, according to the aforementioned conventional multilayered structure (1), adhesiveness between a fluid barrier material such as an ethylene-vinyl alcohol copolymer and an elastomer material such as thermoplastic polyurethane is not in any way considered. In addition, also according to the aforementioned conventional multilayered structure (2), the hard polymer material per se such as an ethylene-vinyl alcohol copolymer, and combination of the same with the flexible polymer material, and the like are not investigated in connection with the interlayer adhesiveness, but only a technique of strengthening the adhesiveness between each layer using a joining layer merely consisting of a hot melt adhesive is disclosed. Therefore, according to these conventional multilayered structure (1) and multilayered structure (2), the interlayer adhesiveness is insufficient, and due to delamination between layers and the like, cracks are likely to be generated on the barrier layer, whereby durability may be deteriorated. As a result, gas barrier properties may be insufficient according to the conventional multilayered structure (1).
On the other hand, a rubber composition containing a butyl rubber, a halogenated butyl rubber or the like as a principal material has been conventionally used for an inner liner provided as an air barrier layer on the inner face of a tire in order to retain an internal pressure of the tire. However, the thickness of the inner liner should be about 1 mm when such a rubber composition is used for an inner liner since the rubber composition containing such a butyl based rubber as a principal material has poor air barrier properties. As a result, the weight of the inner liner reaches approximately 5% of the total weight of the tire, thereby hampering improvement of the fuel efficiency of automobiles, agricultural vehicles, construction vehicles and the like by decreasing the weight of tires.
Accordingly, a technique in which an ethylene-vinyl alcohol copolymer that is superior in gas barrier properties and the like is used for producing an inner liner as described above has been also developed (see, for example, Japanese Unexamined Patent Application, Publication No. H6-40207, and the like). However, when an ethylene-vinyl alcohol copolymer is used as an inner liner, a modulus of elasticity higher than those of butyl rubbers, etc., which have been conventionally used for tires, leads to disadvantages such as fracturing and generation of cracks due to flexion of the tire during running, although significant effects of improving internal pressure retainability of tires can be achieved owing to superior gas barrier properties.
In view of these disadvantages, use of a multilayered structure that includes layers containing an ethylene-vinyl alcohol copolymer as an inner liner may be conceived. However, in order to use the multilayered structure as an inner liner for a pneumatic tire, it is necessary to improve the interlayer adhesiveness and the like described above so as to further enhance the durability such that the tire can endure the environment in use.